Electric discharge lamp

ABSTRACT

An electric discharge lamp comprising: a light-transmissive ceramic lamp vessel ( 1 ); a first and a second current conductor ( 2,3 ) entering the lamp vessel ( 1 ), and each supporting an electrode ( 4,5 ) in the lamp vessel ( 1 ); an ionizable filling comprising a rare gas and a metal halide in the lamp vessel ( 1 ); at least the first current conductor ( 2 ) within the lamp vessel ( 1 ) being halide-resistant, characterized in that the first current conductor ( 2 ) at least substantially comprises a material with an at least substantially isotropic coefficient of thermal expansion, said material preferably being chosen from the group of pentamolybdenum boride disilicide, pentamolybdenum diboride silicide, or a combination of these two materials.

The invention relates to an electric discharge lamp comprising:

-   -   a light-transmissive ceramic lamp vessel;    -   a first and a second current conductor entering the lamp vessel,        and each supporting an electrode in the lamp vessel;    -   an ionizable filling comprising a rare gas and a metal halide in        the lamp vessel; at least the first current conductor within the        lamp vessel being halide-resistant.

Such an electric discharge lamp is known from WO0034980. A first part ofthe first current conductor consists of an halide-resistant material,whereas the second part thereof is made of niobium. Niobium is chosenbecause this material has a coefficient of thermal expansioncorresponding to that of the lamp vessel in order to prevent leakage ofthe lamp. In a particular prior art embodiment said first part is madeof pentamolybdenum trisilicide in order to obviate the risk of leakagein case the sealing compound, being either a ceramic, a glass or acombination thereof, also directly connects the first part of the firstcurrent conductor to the lamp.

A disadvantage of the electric discharge lamp known from the abovePCT-patent application is that if said first part of the first currentconductor is made of pentamolybdenum trisilicide, microcracks may occurin this material when it is sintered, particularly at high temperaturesand/or densities. These microcracks limit the mechanical strength of thefirst current conductor and/or may partly “absorb” the ionisable fillingin the lamp vessel. Furthermore, the micro cracks introduce porositywhich results in leakage, as indicated above.

It is an object of the present invention to obviate this disadvantage.

In order to accomplish that objective, an electric discharge lamp of thetype referred to in the opening paragraph according to the invention ischaracterized in that the first current conductor at least substantiallycomprises a material with an at least substantially isotropiccoefficient of thermal expansion. Said material is preferably chosenfrom the group of Mo₅ (Si,X)₃, wherein X is B, Al, N or C, morepreferably pentamolybdenum diboride silicide. Extensive research hasrevealed that the above mentioned microcracks can be attributed tospecific thermoelastic properties of pentamolybdenum trisilicide leadingto thermal stresses therein. The invention is particularly based on theawareness that the thermoelastic properties of the material used can beimproved and that thermal stresses therein can be prevented by proposinga material that has an at least substantially isotropic coefficient ofthermal expansion, i.e. a coefficient of thermal expansion exhibitingsimilar values in all crystallographic directions of the crystalstructure of the material used. In accordance with the invention,particularly Mo₅ (Si,X)₃, wherein X is B, Al, N or C, more in particularpentamolybdenum diboride silicide, appears to have a (nearly) isotropiccoefficient of thermal expansion, while it simultaneously meets otherrequirements: resistant to the ionizable filling of the lamp,particularly to halide (i.e. they should not be attacked by or reactwith halide or halogen formed therefrom), able to withstand thermalmanufacturing and operating conditions of the lamp, thermally stable upto 2000° C., capable of being attached to the electrode, and havingsufficient electrical conductivity to preclude electrical losses.

In one preferred embodiment of an electric discharge lamp according tothe invention, also the second current conductor at least substantiallycomprises a material with an at least substantially isotropiccoefficient of thermal expansion, said material preferably being chosenfrom the group of Mo₅ (Si,X)₃, wherein X is B, Al, N or C, and morepreferable is pentamolybdenum diboride silicide. This simplifies themanufacture of the lamp, as the same components are used for bothcurrent conductors. As indicated earlier, these materials ideally meetthe requirements of being thermally and chemically stable and having anisotropic coefficient of thermal expansion.

In another preferred embodiment of an electric discharge lamp accordingto the invention, said material adheres to the ceramic material of thelamp vessel at the manufacturing temperature of the lamp. This allows avery compact lamp construction for the following reasons. The prior artlamp as described in the PCT-patent application mentioned earlier makesuse of a sealing compound for sealing the ceramic lamp vessel around thecurrent conductors. As the sealing compound is sensitive to high(operating) temperatures of the lamp, the sealing compound is applied asremote as possible from a central part of the lamp vessel, i.e. at afree end of extended plugs (i.e. elongated end parts) that are connectedto the central part of the lamp vessel by means of sintering The use ofsaid extended plugs is undesirable from a technical point of view: saidplugs function as cooling fins, thereby negatively influencing anoperating temperature in the lamp vessel, whereas capillaries areintroduced in said extended plugs. Part of the lamp filling maycondensate at the location of the capillaries leading to colorinstability of the lamp. In the present preferred embodiment, theclaimed material Mo₅ (Si,X)₃, wherein X is B, Al, N or C is co-sinteredtowards the ceramic lamp vessel at a manufacturing temperature varyingbetween 1500 and 2000° C., so that no separate sealing compound is used,whereas the use of extended plugs being part of the lamp vessel isavoided as well. The present preferred embodiment enables a very compactlamp construction to be achieved, while obviating the prior artdisadvantages discussed above.

In another preferred embodiment of an electric discharge lamp accordingto the invention, the first and the second current conductor each extendfrom a sealing compound sealing the lamp vessel around the currentconductors in a gastight manner to the exterior of the lamp vessel,wherein the lamp vessel has extended plugs in which a respective currentconductor is enclosed, which plugs have a free end where the lamp vesselis sealed by the sealing compound.

The invention will now be explained in more detail with reference to twoFigures illustrated in a drawing, showing preferred embodiments in aside elevation, partly in cross-section.

FIG. 1 shows an electric discharge lamp in accordance with the inventionprovided with a tubular, light-transmissive, ceramic lamp vessel 1 madefrom polycrystalline aluminum oxide, with a first and a second currentconductor 2,3. Said conductors 2,3 enter the lamp vessel 1 opposite eachother and each support a tungsten electrode 4,5 present in the lampvessel 1 and welded to the current conductors 2,3. A ceramic sealingcompound 6 formed in a melting process by 30% by weight of aluminumoxide, 40% by weight of silicon oxide and 30% by weight of dysprosiumoxide, seals the current conductors 2,3 in a gastight manner. The lampvessel 1 has an ionizable filling comprising argon as a rare gas and amixture of sodium, thallium and dysprosium iodide as metal halides.

Both the first and the second current conductor 2,3 each have a firsthalide-resistant part 21,31 within the lamp vessel 1 and, extending fromthe sealing compound 6 to the exterior of the lamp vessel 1, a secondpart 22,32 welded to the first part 21,31. The second part 22,32 of thecurrent conductors 2,3 consists of niobium and is entirely incorporatedin the sealing compound 6 within the lamp vessel 1. In an alternativeembodiment both current conductors 2,3 are each made in one piece of onematerial being Mo₅ (Si,X)₃, wherein X is B, Al, N or C, so that the useof a second part 22,32 of niobium is avoided. This is possible becauseMo5 (Si,X)3, wherein X is B, Al, N or C, has the same coefficient ofthermal expansion as the ceramic material of the vessel 1.

The lamp vessel 1 has narrow end parts or extended plugs 11,12 in whicha respective current conductor 2,3 is enclosed. The plugs 11,12 have afree end 111,121, where the lamp vessel 1 is sealed by the sealingcompound 6. The central part 10 of the lamp vessel 1 is connected bymeans of sintering to the plugs 11,12 via ceramic discs 13. The lampvessel 1 is enveloped by an outer envelope 7 sealed in a gastight mannerand evacuated or filled with an inert gas in order to protect theniobium second parts 22,32 of the current conductors 2,3. The outerenvelope 7 supports a lamp cap 8.

As indicated earlier, the first part 21,31 of the first and the secondcurrent conductor 2,3 consists of a material with an isotropiccoefficient of thermal expansion, which material is preferablypentamolybdenum diboride silicide.

FIG. 2 schematically shows one end of a tubular, light-transmissive,ceramic lamp vessel 1 in accordance with another preferred embodiment,wherein a very compact lamp construction is obtained. The tungstenelectrode 5 present in the lamp vessel 1 is attached (preferably welded)to the first current conductor 2. Said first current conductor 2 isco-sintered to the material of the ceramic lamp vessel 1 at a lampmanufacturing temperature varying between 1500 and 2000° C., withoutusing a separate sealing compound 6 as mentioned in the description ofFIG. 1. The current conductor 2 consists of the same material asindicated with respect to the first current conductor 2 of FIG. 1. Thefirst current conductor 2 of FIG. 2 may form an end wall of the ceramiclamp vessel 1 (FIG. 2 a) or may form an extension of the electrode 5extending through the material of the ceramic lamp vessel 1 (FIG. 2 b).The other end of the ceramic lamp vessel (not shown in FIG. 2) may havethe same construction.

The invention is not restricted to the variant shown in the drawing, butalso includes other embodiments that fall within the scope of theappended claims.

1. An electric discharge lamp comprising: a light-transmissive ceramiclamp vessel (1); a first and a second current conductor (2, 3) enteringthe lamp vessel (1), and each supporting an electrode (4, 5) in the lampvessel (1); an ionizable filling comprising a rare gas and a metalhalide in the lamp vessel (1); at least the first current conductor (2)within the lamp vessel (1) being halide-resistant, characterized in thatthe first current conductor (2) at least substantially comprises amaterial with an at least substantially isotropic coefficient of thermalexpansion.
 2. An electric discharge lamp according to claim 1, whereinsaid material is chosen from the group of Mo₅ (Si, X)₃, wherein X is B,Al, N or C.
 3. An electric discharge lamp according to claim 2, whereinsaid material is pentamolybdenum diboride silicide.
 4. An electricdischarge lamp according to claim 1, wherein also the second currentconductor (3) at least substantially comprises a material with an atleast substantially isotropic coefficient of thermal expansion.
 5. Anelectric discharge lamp according to claim 4, wherein said material ischosen from the group of Mo₅ (Si, X)₃, wherein X is B, Al, N or Cpreferably is pentamolybdenum diboride silicide.
 6. An electricdischarge lamp according to claim 1, wherein said material isco-sintered to the ceramic material of the lamp vessel (1) at amanufacturing temperature of the lamp.
 7. An electric discharge lampaccording to claim 1, wherein the first and the second current conductor(2, 3) each extend from a sealing compound (6) sealing the lamp vessel(1) around the current conductors (2, 3) in a gastight manner to theexterior of the lamp vessel (1), and wherein the lamp vessel (1) hasextended plugs (11, 12) in which a respective current conductor (2, 3)is enclosed, which plugs (11, 12) have a free end (111, 112) where thelamp vessel (1) is sealed by the sealing compound (6).